Direct hydrocarbon fuel cells

ABSTRACT

A method of using a hydrocarbon fuel to operate a solid oxide fuel cell, such a cell comprising one or more components which can be prepared using a centrifugal deposition technique of the type described herein.

[0001] This application is a continuation-in-part of and claims prioritybenefit from application Ser. No. 09/833,209 filed on Apr. 10, 2001,which is a continuation-in-part of and claims priority from applicationSer. No. 09/441,104 filed on Nov. 16, 1999, issued as U.S. Pat. No.6,214,485 on Apr. 10, 2001.

FIELD OF THE INVENTION

[0002] This invention relates generally to fuel cells and assemblies,more particularly, cells and components thereof as can be configured foruse with hydrogen fuel or the direct oxidation of hydrocarbons.

BACKGROUND OF INVENTION

[0003] Fuel cells are promising electrical power generationtechnologies, with key advantages including high efficiency and lowpollution. Most potential near-term applications of fuel cells requirethe use of hydrocarbon fuels such as methane, for which a supplyinfrastructure is currently available. However, fuel cells typicallyoperate only with hydrogen as the fuel. Thus, current demonstrationpower plants and planned fuel-cell electric vehicles must include ahydrocarbon fuel reformer to convert the hydrocarbon fuel to hydrogen.Fuel cells that could operate directly on hydrocarbon fuels wouldeliminate the need for a fuel reformer, providing considerable systemand economic advantages and presumably improving the viability of thetechnology.

[0004] Prior art fuel cells utilizing hydrocarbon fuels directly haveencountered significant problems. For example, direct-methanol polymerelectrolyte fuel cells produce relatively low power densities andrequire prohibitively large Pt loading of the anodes. In addition,methanol can permeate the electrolyte. See, for instance, Ren, X.,Wilson, M. S. and Gottesfeld, S. High performance direct methanolpolymer electrolyte fuel cells. J. Electrochem. Soc., 143, L12-L14(1996); and Wang, J., Wasmus. S. and Savinell, R. F. Evaluation ofethanol, 1-propanol, and 2-propanol in a direct oxidationpolymer-electrolyte fuel cell a real-time mass spectrometry study. J.Electrochem. Soc., 142, 4218-4224 (1995). Furthermore, only alcoholfuels appear feasible with this approach.

[0005] Alternatively, prior art solid oxide fuel cells (SOFCs) canutilize hydrocarbons directly via internal or external reforming. Inthis approach, a hydrocarbon fuel (e.g., methane) is combined with H₂Oand/or CO₂, which are typically obtained by recirculating the fuel cellexhaust, and introduced directly to the SOFC anode. Commonly usedNi-based anodes provide the catalyst for the endothermic reformingreactions,

CH₄+H₂O=3H₂+CO ΔH°₂₉₈=206 kJ/mol CH₄  (1)

CH₄+CO₂=2H₂+2 CO ΔH°₂₉₈=247 kJ/mol CH₄  (2)

[0006] However, maintaining appropriate gas composition and temperaturegradients across a large area SOFC stack is challenging. See, Janssen,G. J. M., DeJong, J. P., and Huijsmans, J. P. P. Internal reforming instate-of-the-art SOFCs. 2nd European Solid Oxide Fuel Cell Forum,163-172, Ed. by Thorstense, B. (Oslo/Norway, 1996); and Hendriksen, P,V., Model study of internal steam reforming in SOFC stacks. Proc. 5thInt. Symp. on Solid Oxide Fuel Cells, 1319-1325, Ed. by U. Stimming,S.C. Singhal, H. Tagawa, and W. Lehnert (Electrochem, Soc., Pennington,1997).

[0007] For instance, if the reforming reactions are slow, theninsufficient H₂ is supplied to the SOFCs. On the other hand, fastreforming reactions cause cooling localized near the fuel inlet, leadingto poor cell performance, and possible cell fracture. Thus, current SOFCstacks of the prior art do not take full advantage of internalreforming; rather, they employ a combination of ≈75% external and 25%internal reforming of hydrocarbon fuels. See, Ray, E. R. WestinghouseTubular SOFC Technology, 1992 Fuel Cell Seminar, 415-418 (1992).

[0008] SOFCs can in principle operate directly by use of a hydrocarbonfuel. This approach would be desirable since it eliminates the problemswith internal reforming mentioned above, and the theoretical maximumfuel efficiency is as good or better than that for reforming. However,prior art attempts with SOFCs operating at temperatures T_(c)=900-1000°C. with methane fuel have been less than satisfactory: either powerdensities were very low or carbon deposition was observed. See, Putna,E. S., Stubenrauch, J., Vohs, J. M. and Gorte, R. J. Ceria-based anodesfor the direct oxidation of methane in solid oxide fuel calls, Langmuir11, 4832-4837 (1995); and Aida, T., Abudala, A., Ihara, M., Komiyama, H.and Yamada, K. Direct oxidation of methane on anode of solid oxide fuelcell. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 801-809, Ed. byDokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C, (Electrochem.Soc. Pennington, 1995).

[0009] Recently, SOFCs have been developed to produce high powerdensities with hydrogen at reduced temperatures, T_(c)=600-800° C. See,Huebner, W., Anderson, H. U., Reed, D. M., Sehlin, S. R. and Deng, X.Microstructure property relationships of NiZrO₂ anodes. Proc. 4th Int.Symp. on Solid Oxide Fuel Cells, 696-705, Ed. by Dokiya, M., Yamamoto,O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995);daSouza, S., Visco, S J. and DeJonghe, L. C. Thin-film solid oxide fuelcell with high performance at low-temperature. Solid State Ionics 98,57-61 (1997); Fung, K-Z., Chen, J., Tanner, C. and Virkar, A. V. Lowtemperature solid oxide fuel cells with dip-coated YSZ electrolytes.Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 1018-1027, Ed. byDokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem.Soc. Pennington, 1995); Minh, N. Q. Development of thin-film solid oxidefuel cells for power generation applications. Proc. 4th Int. Symp. onSolid Oxide Fuel Cells, 138-145, Ed. by Dokiya, M., Yamamoto, O.,Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995);Godickemeier, M., Sasaki, K. and Gauckler, L. J. Current-voltagecharacteristics of fuel cells with ceria-based electrolytes. Proc. 4thInt. Symp. on Solid Oxide Fuel Cells, 1072-1081, Ed. by Dokiya, M.,Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc.Pennington, 1995); Tsai, T. and Barnett, S. A. Increased solid-oxidefuel cell power density using interfacial ceria layers. Solid StateIonics 98, 191-196 (1997); and Tsai, T., Perry, E. and Barnett, S.Low-temperature solid-oxide fuel cells utilizing thin bilayerelectrolytes. J. Electrochem. Soc., 144, L130-L132 (1997). However,until recently, such systems were not considered or used fordirect-hydrocarbon operation, because carbon deposition reaction ratesdecrease with decreasing temperature.

[0010] SOFCs and related stacking configurations have undergoneconsiderable development over the past decade. Tubular-cell-basedtechnologies appear to be a promising approach for SOFC stacking.Tubular stacks avoid sealing and manifolding problems inherent to planarstacks, but take a large volume for a given cell active area and canshow significant ohmic losses related to current transport around thetube circumference through the (La,Sr)MnO₃ (LSM) cathode. Anotherproblem is the relatively poor mechanical toughness of LSM.[N. M.Sammes, R. Ratnaraj, and C. E. Hatchwell, Proceedings of the 4thInternational Symposium on Solid Oxide Fuel Cells, Ed. By Dokiya, O.Yamamota, H. Tagawa, and S.C. Singhal (Electrochemical Society,Pennington, 1995) p. 952. B. Krogh, M. Brustad, M. Dahle, J. L.Eilertsen, and R. Odegard, Proceedings of the 5th InternationalSymposium on Solid Oxide Fuel Cells, Ed. By U. Stimming, S.C. Singhal,H. Tagawa, and W. Lehnert (Electrochemical Society, Pennington, 1997) p.1234.] This is typical of SOFC ceramic materials, which are optimizedfor electrical properties rather than mechanical toughness.

[0011] Alternatively, planar stacks can provide higher power-to-volumeratios than tubular stacks, but are not as mechanically robust as tubesand require excellent seals. Another problem with many planar stackdesigns is that they require pressure contacts between separate SOFC andinterconnect plates. This places stringent requirements on the flatnessof large-area ceramic plates, making manufacturing difficult andexpensive. Furthermore, there are often relatively high resistancesassociated with these contacts, which deleteriously affect stackperformance. It is clear that a choice between tubular and planar stacksinvolves trade-offs. Even so, the disadvantages associated with eachrespective approach present obstacles for effective use of SOFCs andsuggest a new direction is needed to better utilize and benefit fromthis technology.

OBJECTS OF THE INVENTION

[0012] There are a considerable number of problems and the deficienciesassociated with the use of hydrocarbons with solid oxide fuel cells.There is a demonstrated need for the use of such fuels in an efficient,economical fashion so as to improve the viability of the relatedtechnology.

[0013] Accordingly, it is an object of the present invention to providevarious solid oxide fuel cells and/or components which can be used withhydrocarbon fuels thereby overcoming various deficiencies andshortcomings of the prior art, including those outlined above. It willbe understood by those skilled in the art that one or more aspects ofthis invention can meet certain objectives, while one or more otheraspects can meet certain other objectives. Each objective may not applyequally, in all instances, to every aspect of the present invention. Assuch, the following objects can be used in the alternative with respectto any one aspect of the present invention.

[0014] It can be an object of the present invention to increase the rateof hydrocarbon oxidation so as to increase and/or otherwise provideuseful power densities. Such densities can be increased and/or providedutilizing various catalytic metals in the fabrication of fuel cellanodes, such anodes as can be used in conjunction with a ceria material.

[0015] It can be an object of the present invention to utilize solidoxide fuel cells and/or components thereof for low temperature directhydrocarbon oxidation.

[0016] It can also be an object of the present invention to providevarious anodes and related cellular components having small particlesize obtainable by sputter deposition processes and/or relatedfabrication techniques.

[0017] It can also be an object of the present invention to provide amethod for hydrocarbon oxidation, at a temperature lower than 800° C.and/or at a temperature for a specific hydrocarbon whereby there is nocarbon deposition.

[0018] It can also be an object of the present invention to improve theviability of solid oxide fuel cells, both those described herein as wellas those otherwise known in the art, through use of a unique assembly ofsuch cells having a configuration and/or geometry of the type describedherein. In particular, it is an object of this invention to provide acell geometry/configuration whereby all active fuel cell components andinterconnects are deposited as thin layers on an electrically insulatingsupport.

[0019] It can also be an object of the present invention, in conjunctionwith one or more of the preceding objectives, to provide ageometry/configuration for an assembly of solid oxide fuel cells wherebyassembly design and choice of support material can enhance mechanicaldurability and thermal shock resistance. A related objective is todecrease overall material cost by providing all cell-active materials inthin layer/film form.

[0020] It can also be an object of the present invention to improve anumber of solid oxide fuel cell performance or function parametersthrough integration of the cell components and interconnects on a commonsupport, such advantages including reduction of electrical resistancesand interconnect conductivity requirements. As described more fullybelow, such integration can be accomplished through use of the thinfilm/layer configurations and related geometries described herein.

[0021] Another object of this invention is to provide a cell assemblyconfiguration suitable for SOFCs of the type described herein,especially those operable at low temperatures for direct oxidation ofhydrocarbon fuels, such cells as can be prepared to preferentiallyincorporate the catalytic metal anodes of this invention.

[0022] Other objects, features, benefits and advantages of the presentinvention will be apparent from the following summary and descriptions,and will be readily apparent to those skilled in the art made aware ofthis invention and having knowledge of various and solid oxide fuelcells in the use of hydrocarbon fuels. Such objects, features, benefitsand advantages will be apparent from the above as taken in conjunctionwith the accompanied examples, tables, data and all reasonableinferences to be drawn therefrom.

SUMMARY OF THE INVENTION

[0023] The present invention provides for the operation of SOFCs usinghydrocarbon fuels over a range of temperatures. High power densitieswere obtained via direct hydrocarbon use, without carbon deposition. Theresults shown herein can be extendable to fuel cell stack operation. Inparticular and without limitation, the present invention demonstratesthe feasibility of direct hydrocarbon electrochemical oxidation in novellow-temperature solid oxide fuel cells. For instance, power densities upto 0.37 W/cm² were measured for single cells that were operated at 650°C. with atmospheric-pressure air as the oxidant and pure methane as thefuel. The measured power densities are competitive with fuel cellsoperated on hydrogen. As discussed more fully below, such results can beobtained at low operating temperatures (T_(c)<800° C.) and/or byincorporating ceria in the anodes of such cells.

[0024] In part, the present invention is a method of using a solid oxidefuel cell for direct hydrocarbon utilization. The method includes (1)providing a catalytic metal anode and a ceria material contacting theanode and (2) introducing a hydrocarbon fuel to said cell, said fuelabsent carbon dioxide and/or water in an amount sufficient to convertthe hydrocarbon fuel to hydrogen under cell operating conditions. Assuch, the method is absent a hydrocarbon reforming stage.

[0025] The anode of the solid oxide fuel cell can be constructed using ametal catalytic for the cracking of hydrocarbons. Such a metal includesbut is not limited to Pt, Ru, Pd, Fe, Co and Ni present at weightpercentages of the type described elsewhere herein. Various embodimentsof the present invention can also include a doped lanthanum chromite,doped strontium titanate, or other electronically-conducting oxides.Various other preferred embodiments, including oxidation of lowermolecular weight hydrocarbons, utilize nickel.

[0026] In preferred embodiments of the present invention, the ceriamaterial includes a dopant. Such dopants include but are not limited tovarious oxides of yttrium, gadolinium and samarium. Highly preferredembodiments include a yttria-doped ceria, having the stoichiometricrelationship of (Y₂O₃)_(x)(CeO₂)_(1−x), where “x” can be about 0.1 toabout 0.25. One such embodiment is (Y₂O₃)_(0.15)(CeO₂)_(0.85), althoughother such stoichiometries would be known, to those skilled in the artmade aware of this invention, to provide a similar or comparablefunctional result.

[0027] With reference to use of a nickel metal and only by way ofexample, the catalytic anode can comprise a nickel composite. Such acomposite can further include ceria and/or zirconia materials or layersof such materials used in conjunction with the nickel metal. Zirconiacan be introduced to such a composite as an electrolyte adjacent toand/or contacting the anode. In preferred zirconia embodiments, variousdopants can also be utilized, such dopants including but not limited tocalcium, scandium, and yttrium. As would be well-known to those skilledin the art and made aware of this invention, other electrolytes can beused, including ceria, strontium-doped lanthanum gallium magnesiumoxide, any of which can be doped as discussed elsewhere herein.

[0028] The method of the present invention provides for directutilization and/or oxidation of hydrocarbon fuels, substantially withoutany reformation reaction. Fuels especially suitable for use hereininclude, without limitation, C₁-C₈ alkanes, and the correspondingalcohols. Likewise, combinations of such hydrocarbons can be utilizedwith good effect, some mixtures for the purpose of approximating naturalgas compositions.

[0029] Accordingly, this invention can also include use of suchhydrocarbon fuels with a variety of catalytic metal-electrolytecomposites, anodes and/or related fuel cells. In the past, conventionalSOFCs with thin yttria stabilized zirconia (YSZ) electrolytes on Ni-YSZanode supports were shown to provide power densities of 1 W/cm² orhigher at operating temperatures of ≈800° C. Both the high powerdensities and the reduced operating temperatures are advantageous forpractical SOFC power plants. While most of the work on such cells hasbeen with hydrogen fuel, high power density operation of anode-supportedcells was recently demonstrated using methanol. See, Y. Jiang and A. V.Virkar, J. Electrochem. Soc. 148 (2001) A706. And, as discussed herein,SOFCs can also work with dry or slightly (3%) humidified methane usingceria-containing anodes to increase methane oxidation rates.Alternatively, SOFCs with nickel-scandia stabilized zirconia (Ni-ScSZ)anodes have been operated with low-humidity methane at highertemperatures, e.g. 1000° C., without coking. See, e.g., K. Ukai, Y.Mizutani, Y. Kume, and O. Yamamoto, in: H. Yokokawa, S. C. Singhal(Eds.), Solid Oxide Fuel Cells VII, Electrochemical Society, Pennington,N.J., 2001, p. 375. The higher operating temperatures and theScSZ-containing anodes apparently increased methane oxidation rates,while coking was suppressed by the high oxygen ion flux arriving at theanode during cell operation.

[0030] However, as shown below, conventional Ni-YSZ anode cells can alsooperate on hydrocarbons such as methane. (See, i.e., examples 6 and23-29.) Further, as natural gas compositions comprise methane incombination with various mixtures of higher hydrocarbons, a range ofsuch combinations and/or natural gas compositions can also be utilizedwith conventional anodes and/or related SOFCs. Cells demonstrating thisutility were prepared, for purposes of illustration, using eitherlanthanum strontium cobalt iron oxide-gallium doped ceria (LSCF-GDC) orLSM-YSZ cathodes. Power densities of nearly 1.0 W/cm² at open circuitvoltages of up to 1.165 V were obtained with both methane and naturalgas fuels at 800° C., as shown below.

[0031] As mentioned above and demonstrated below, the present inventionalso includes a method of using a hydrocarbon fuel to operate a solidoxide fuel cell. Such a method includes (1) providing a solid oxide fuelcell having an anode comprising a catalytic metal, such an anode having,optionally, a ceria material of the type described herein in contacttherewith; and (2) introducing a hydrocarbon fuel to the anode, the fuelabsent carbon dioxide, water and/or oxygen in an amount sufficient toconvert the fuel to hydrogen under cell operating conditions. As such,as also described above, such a method is substantially absent ahydrocarbon reforming stage. The anode of the solid oxide fuel cell canbe constructed as otherwise provided herein. Preferably, the anodecomprises nickel present at weight percentages of the type describedelsewhere herein and/or as would be understood by those skilled in theart of fabricating solid oxide fuel cells. As a departure from variousother embodiments of this invention, such an anode component can bedistinguished as without use of a ceria material in contact therewith.Reference is made, for instance, to the comparative test results ofexample 6 wherein a representative hydrocarbon fuel (e.g., methane) wasutilized in conjunction with a more typical or conventional anode of theart, one including a composite of nickel and yttria-stabilized zirconia,operated in conjunction with a corresponding electrolyte. As describedelsewhere herein and as would be understood by those skilled in the art,such a cell can be arranged and configured for operation in a stack orbattery of such cells, and operated with a hydrocarbon fuel.

[0032] As demonstrated below, a range of hydrocarbon fuels can be usedin conjunction with this invention. Such fuels include, withoutlimitation, about C₁, C₂, C₃ or C₄ . . . about C₁₀ hydrocarbons and/oralkanes, either alone (e.g., methane or ethane) or as provided in thecontext of various possible combinations or mixtures (e.g., a naturalgas composition). More generally, such fuels include those which can bevaporized or dispersed, or have sufficient vapor pressures, under anodecompartment temperatures. Other fuels utilized include ethers andalcohols corresponding to such hydrocarbons, such as but not limited todimethyl and diethyl ether, methanol and ethanol. JP8, a kerosene-typemixture of hydrocarbons, is also used with good effect.

[0033] More specifically, in contrast to the prior art and use ofhydrogen, this invention can also include use of a hydrocarbon fuel toincrease the open circuit voltage of a solid oxide fuel cell. Such amethod includes (1) providing a solid oxide fuel cell as describedabove; (2) introducing a hydrocarbon fuel to the anode, to operate thecell; and (3) increasing the operating temperature of the cell. Enhancedperformance is especially notable and can be observed at operatingtemperatures above about 600° C. Reference is made to Examples 28-29 andthe corresponding data and figures. Again, a preferred anode cancomprise nickel, but other such catalytic metals can be used with goodeffect. Likewise, such an anode can be preferably used in conjunctionwith a zirconia material introduced during anode fabrication so as toprovide a metal composite and/or an electrolyte adjacent thereto.Enhanced anode/cell performance is demonstrated below using,alternatively, methane and a natural gas composition. However, as wouldbe understood by those in the art and made aware of this invention,comparable benefits are available with other hydrocarbon fuels of thesort described herein.

[0034] Regardless of fuel utilized, the present invention also includesa method of preparing or fabricating a solid oxide fuel cell component.Such a method includes (1) preparing a slurry of a material from which acomponent is to be fabricated; (2) providing a support or substrate withthe slurry in a centrifuge apparatus; and (3) applying centrifugal forceto the slurry and support/substrate, the force sufficient to deposit thecomponent material on the support/substrate. Such a method can be usedin the fabrication of a variety of solid oxide fuel cell components. Invarious preferred embodiments, an anode material and/or an electrolytematerial can be deposited in sequence upon a suitable support/substrate.In various other embodiments, the supporting substrate comprises ananode or cathode component, for deposition of an electrolyte slurrythereon. Regardless, multiple depositions can be used to control orenhance component dimension.

[0035] The liquid medium of such a slurry can include an alcohol, waterand a corresponding variety of mixed solvent systems. Various otherorganic or mixed solvents can be used, as would be understood by thoseskilled in the art, depending upon a particular component material.Accordingly, a variety of ketones and other polar organic solvents canbe used alone or with a corresponding range of mixed solvent systems.Drying of the deposited component (e.g., anode and/or electrolyte) canbe used to remove the slurry liquid medium, followed by sinteringleaving a uniform material coating of component density suitable for usein conjunction with solid oxide fuel cell.

[0036] In part, the present invention is also a method of using a ceriamaterial to increase hydrocarbon oxidation rates in a solid oxide fuelcell. The inventive method includes (1) providing a solid oxide fuelcell having an anode composite of a catalytic metal and a ceria layer,(2) operating the cell at a temperature less than about 800° C., (3)introducing a hydrocarbon fuel directly to the anode and (4) sorbingoxygen with the ceria layer for transfer to the anode for hydrocarbonoxidation. Solid oxide fuel cells can be constructed and/or fabricatedusing methods and techniques well-known to those skilled in the art,together with use of the cell components otherwise as described morefully herein. In preferred embodiments, the hydrocarbon is methane,ethane or a combination thereof, although other fuels can include thosepreviously discussed. Irrespective of the choice of hydrocarbon fuel,preferred embodiments of such a method include operating the cell,together with its anode, at a temperature between about 500° C. andabout 700° C.

[0037] In part, the present invention is also a method for suppressingand/or eliminating carbon deposition during electrochemical oxidation ofa hydrocarbon in a fuel cell. The method includes (1) providing a solidoxide fuel cell anode composite of a catalytic metal and a ceria layer,(2) operating the cell at a temperature less than about 800° C., (3)introducing the hydrocarbon directly to the anode and (4) oxidizing thehydrocarbon at the anode substantially without carbon deposition on theanode. As with other aspects of the present invention, this method canbe effected using fuel cells of the prior art and/or as constructedand/or fabricated as elsewhere described herein. In particular, butwithout limitation, the anode comprises a catalytic metal selected fromthe group consisting of Pt, Ru, Pd, Fe, Co and Ni. Regardless, preferredembodiments include introducing oxygen electrochemically at the anode ata rate and pressure sufficient to react the oxygen with any elementalcarbon present, whereby carbon monoxide disproportionation and/orhydrocarbon pyrolysis are inhibited. While operating pressures less than800° C. provide the desired effect, such embodiments can be employedbeneficially at lower temperatures, typically between about 500° C. andabout 700° C., depending on the anode material and/or the hydrocarbonoxidized.

[0038] In part, the present invention is also an anode for directhydrocarbon oxidation in a solid oxide fuel cell. The anode includes (1)a composite having a catalytic metal and a ceria material (2) such thatthe metal is present in an amount less than about 60 weight percent ofthe anode. Catalytic metals of the present invention include those knownto those skilled in the art as useful for the cracking and/or oxidationof hydrocarbons. In preferred embodiments, such a metal can includethose of the type described more fully above. Regardless, the ceriamaterial can be used with or without a dopant. In any event, the anodeof this invention is substantially without carbon deposits under celloperating conditions. At lower metal levels, the present inventioncontemplates use of a current collector as needed to supplementconductivity.

[0039] High power density SOFCs and related methods of this inventionoperate by direct electrochemical hydrocarbon oxidation without carbondeposition. The anodes described herein provide for rapid hydrocarbonelectrochemical oxidation rates. The results, confirmed with a simplethermodynamic analysis, show that SOFC stacks can be operated in thetemperature range from ≈500 to 700°-800° C. without carbon deposition.Direct oxidation provides a desirable method for utilizing a variety ofhydrocarbon fuels, avoiding the difficulties associated with reforming.Indeed, this may be the only feasible approach for low-temperatureSOFCs, since extrapolation of internal reforming rate data below 750° C.suggests that reforming rates become prohibitively small.

[0040] In part, the present invention is a solid oxide fuel cell stackand/or composite configuration, including: (1) a substantially planar,electrically-insulating substrate; (2) a plurality of cathode componentson the substrate, each cathode component spaced one from another; (3) anelectrolyte on and between each cathode; (4) a plurality of anodecomponents, each anode spaced one from another and corresponding innumber to the plurality of cathode components; and (5) an interconnectcomponent contacting the portion of each cathode component and a portionof each corresponding anode component. Such a configuration provides forfuel and oxidant cavities as shown, for instance, in FIG. 7B.Alternatively, a plurality of anode components can be deposited on asubstrate, with a corresponding number of cathode componentsinterconnected therewith.

[0041] In preferred embodiments, the solid oxide fuel cell compositeincludes a catalytic metal anode and a ceria material contacting theanode, as described more fully above, for direct hydrocarbon oxidation.In such embodiments, the catalytic metal includes, but is not limitedto, Pt, Ru, Pv, Fe, Co and Ni present at weight percentages of the typedescribed elsewhere herein. Other embodiments, preferred or otherwise,can be utilized with comparable effect depending upon the type of fuel.Regardless, preferred embodiments of such fuel cell composites include adoped ceria material. Highly preferred embodiments include ayttria-doped ceria having a stoichiometric relationship such as thatprovided elsewhere herein.

[0042] In part, the present invention can also include a solid oxidefuel cell assembly, including: (1) a substantially planar array of fuelcells on a substrate, each cell having cathode, anode, electrolyte andinterconnect component structures, with each component structure of eachcell having a sub-planar arrangement of one to another; (2) an oxidantcavity adjacent the substrate; and (3) a fuel cavity adjacent thesub-planar anode arrangement. As discussed above, and as furtherdescribed in example 14, an assembly configured with anodes on asubstrate will provide a converse cavity placement; i.e., a fuel cavityadjacent the substrate and an oxidant cavity adjacent the anodes. Fuelsand oxidants useful with such cells and related assemblies are asdescribed herein or otherwise known in the art. Likewise, the requisitecavities and supporting cellular/assembled structures will be understoodupon consideration of various aspects of this invention.

[0043] A preferred embodiment of such an assembly is illustrated inseveral of the following figures. In particular, a plurality of suchplanar arrays, configured with the corresponding oxidant and fuelcavities can provide a stacking configuration such as that portrayed inFIG. 7B.

[0044] In part, the present invention is also a method of constructing aseries of solid oxide fuel cells, such cells as can be used inconjunction with the composites and/or assemblies described above. Sucha method, without limitation, includes one or more of the followingconstructions: (1) providing a substrate with masks aligned thereon in apredetermined pattern; (2) placing/depositing a first electrode materialon the substrate; (3) re-aligning the masks on the first electrodematerials, one mask on each such first electrode material; (4)placing/depositing an electrolyte material on the first electrodematerial; (5) removing the masks and placing/depositing an interconnectmaterial on the first electrode material; and (6) re-aligning the maskson the electrolyte and placing/depositing a second electrode material onthe electrolyte.

[0045] Such electrode/electrolyte and/or interconnect components can beprepared and integrated on a substrate and with one another as describedherein, using thin-film/layer techniques of the prior art, suchtechniques and straight-forward modifications thereof as would beunderstood by those skilled in the art made aware of this invention.Successive masking, deposition, and unmasking procedures can be employedto deposit/print cathode, electrolyte, interconnect and anode componentson a suitable insulating substrate. Such procedures or fabrication stepswould also be known to those skilled in the art, modified as necessaryto accommodate use of the component materials described herein or tootherwise achieve the functional and/or performance characteristicsdesired. Fabrication in this manner on a suitable substrate, provides aplanar composite, array and/or assembly of solid oxide fuel cellswherein the cells are integrated one with another. (See, for example,FIG. 7A.) The cellular stacking geometries of this invention have,therefore, the capacity to be two- or three-dimensional. Such procedurescan be viewed as analogous to various thin-film/layer techniques used inthe fabrication of micro- and nano-dimension integrated devices, hencethe reference to integration and integrated stacks.

[0046] Individual planar, integrated assemblies can be mounted one aboveanother and between structural components described elsewhere herein andas necessary to provide a functional fuel cell. Such end-plates/caps,fuel feed tubes and associated non-conductive seals are of well-knownmaterial choice and construction, the design of which can be as shown inFIG. 7B or, in accordance with this invention, as necessary to providethe desired performance property or parameter.

[0047] As discussed above, SOFCs are a very promising energy conversiontechnology for utilization of fossil fuels and hydrocarbons producedtherefrom. The present invention introduces a novel stacking geometrydevised to enhance the benefits available from this technology. Thegeometry involved has all active SOFC components and the interconnectdeposited as thin layers on an electrically insulating support. Thisconfiguration allows the choice of a support material that providesoptimal mechanical toughness and thermal shock resistance. The supportscan be in the form of flattened tubes, providing relatively highstrength, high packing densities, and minimizing the number of sealsrequired. The integration of SOFCs and interconnects on the same supportprovides several other advantages including the reduction of electricalresistances associated with pressure contacts between the cells andinterconnects, relaxation of fabrication tolerances required forpressure contacts, reduction of ohmic losses, and reduction ofinterconnect conductivity requirements. The materials used in theintegrated stacks of this invention can be similar to or the same usedwith conventional SOFCs, and long-term stable operation will beachievable. Use of thin layer cell-active components helps to loweroverall material costs.

[0048] Without limitation as to the scope of this invention andirrespective of the fuel (hydrogen or hydrocarbon) used, the followingprovides several advantages, attributes and/or aspects pertaining to oneor more embodiments of the stacking configurations, geometries and/orassemblies described herein.

[0049] 1. The support is not an electrically active part of the stack;it can be designed chosen for optimal mechanical properties.

[0050] 2. There are no separate interconnect pieces, and a reduction inthe number of seals is possible. The cells can be fabricated in the formof flattened tubes, such that a seal-less design similar to tubularstacks can be implemented while retaining the high power-to-volumeratios of planar stacks.

[0051] 3. Because the SOFC components and interconnects are in intimatecontact, electrical losses related to pressure contacts are greatlyreduced, improving stack performance.

[0052] 4. Because no separate interconnect pieces and fewer gas-flowchannels are required, there is a reduction in stack volume and weight.

[0053] 5. Large integrated stack elements can be made by increasing thenumber of cells: there is no need to make very large area cells.

[0054] 6. Considerable flexibility is provided by way of stack design:for example, individual cell sizes can be varied slightly to account forspatial variations in gas composition and temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055]FIG. 1A illustrates cell voltage and power density vs. currentdensity for an SOFC operated on air and methane; FIG. 1B providesvariations in operating temperature for the cell of FIG. 1A and therespective voltage and power densities observed therewith and as shownin FIG. 1A. The measurements were collected in atmospheric pressure air,and methane fuel was supplied at ˜50 cm³ STP min⁻¹.

[0056]FIG. 2 shows a comparison of electrode impedance spectra forNi-YSZ/YDC anodes measured at 600° C. in (a) 97% CH₄+3% H₂O, (b) 3%H2+3% H₂O+94% Ar, and (c) 97% H₂+3% H₂O.

[0057]FIG. 3 compares impedance spectroscopy results for Ni-YSZ andNi-YSZ/YDC anodes in 97% CH₄+3% H₂O at 600° C.

[0058]FIG. 4 shows CO/CO₂ ratios calculated at equilibrium withgraphitic carbon (•P_(CO)+P_(CO2), 0.1 atm; and ▪P_(CO)+P_(CO2), 0.2atm) based on reaction (5), in Example 7.

[0059]FIG. 5 is a table showing conditions that limit and promote carbondeposition, as compared to various Ni-YDC anodes and the parentheticalreference to Ni weight percent therein. While such percentages are shownwith respect to ethane and nickel-based anodes, the same can be extentedwith comparable effect to other anode metals and the direct oxidation ofother hydrocarbons, as would be well known to those skilled in the artand made aware of this invention.

[0060]FIG. 6 compares cell power density vs. current density for a solidoxide fuel cell operated on air and ethane.

[0061]FIG. 7A shows an enlarged schematic view of an integrated stackelement showing the cell-interconnect geometry.

[0062]FIG. 7B illustrates a hydrogen (or hydrocarbon) flow cavity can becreated between two such stack elements by sealing at the edges. Theaddition of end-caps that include fuel-feed tubes results in a fullysealed geometry.

[0063]FIG. 8 provides a comparison of various performance properties andparameters associated with a typical solid oxide fuel cell (SOFC), asdescribed, and an integrated solid oxide fuel cell (ISOFC) of thisinvention.

[0064]FIG. 9 demonstrates another aspect of the present invention, lowlateral resistance loss and reduction thereof with cell width, as can beillustrated with Ni-YSZ and LSM cathodes.

[0065]FIG. 10 is a schematic illustration of stacked integrated solidoxide fuel cells, in accordance with this invention, showing inparticular, the structural relationship and placement of various cellassembly components such as current collectors and fuel tubes.

[0066]FIG. 11 provides and compares representative materials for thecell components of this invention.

[0067]FIG. 12 is an SEM fracture cross-sectional image of a typicalthin-electrolyte SOFC showing LSM-YSZ substrate, YSZ electrolyte, andNi-YSZ anode (the thin YDC layer is not visible).

[0068]FIG. 13 is an optical micrograph showing a portion of a patternedNiO-YSZ anode layer on a CSZ support.

[0069]FIG. 14 is an SEM micrograph of the top surface of a sintered YSZlayer.

[0070]FIG. 15 is an SEM micrograph showing the masked edge of a YSZelectrolyte on Ni-YSZ. The Ni-YSZ (lower part) can be distinguished fromthe YSZ layer (upper part) by its smaller grain size.

[0071]FIG. 16 provides a cross sectional SEM image of a SOFC with aLSM-YSZ cathode.

[0072]FIG. 17 plots graphically voltage and power density versus currentdensity for the cell with a LSCF-GDC cathode (operated using humidifiedhydrogen).

[0073]FIG. 18 plots graphically voltage and power density versus currentdensity for the cell with a LSM-YSZ cathode (operated using humidifiedhydrogen).

[0074]FIG. 19 compares the present maximum power density results withliterature data.

[0075]FIG. 20 compares impedance spectra from a cell with a LSCF-GDCcathode at different temperatures (operated using humidified hydrogen).

[0076]FIG. 21 plots voltage and power density versus current densitymeasured with humidified methane fuel, for a SOFC with LSM-YSZ cathode.

[0077]FIG. 22 plots voltage and power density versus current densitymeasured with humidified natural gas fuel, for a SOFC with LSM-YSZcathode.

[0078]FIG. 23 plots current density at 0.6V versus time for a SOFC witha LSCF-GDC cathode operated at 700° C. using methane.

[0079]FIG. 24 shows an SEM-EDX spectrum taken from the surface of aNi-YSZ anode after testing in methane at 700° C. for 90 hours.

[0080]FIG. 25 plots open circuit voltages measured in both humidifiedhydrogen and methane, for a SOFC with LSCF-GDC cathode. Shown forcomparison are values calculated using the Nernst equation assumingdifferent anode reactions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0081] One schematic perspective view of an SOFC stack, in accordancewith this invention, is shown in FIGS. 7A and 7B. An array of SOFCs 12,connected in series by thin-film interconnects 22, are deposited onto aporous, insulating support 14. The layers 12 can be deposited in theorder: electrode (cathode or anode), electrolyte, interconnect, andelectrode (cathode or anode). The patterning is such that electrolyte 20and interconnect 22, which are both dense layers, are continuous andform a gas-tight seal over the entire surface. With respect to FIGS. 7Aand 7B, the electrodes, cathode 16 and anode 18 are porous layersseparated by electrolyte 20, and connected by interconnect 22.Alternatively, and as elsewhere described more fully, anodes can bepositioned on the substrate and interconnected with cathodes.

[0082] By contrast, in conventional planar stacks, the current flowsbetween separate SOFC and interconnect pieces: the stacking order ishydrogen (fuel)-SOFC-oxygen-interconnect-hydrogen (fuel)-SOFC, and soforth. The polarity of the SOFCs must be maintained in order foradditive voltages. In the stacks 10 of this invention, however, thecurrent flows across the integrated stack elements. There is norequirement that the pieces be stacked with the same polarity, such thatthe cells can be mounted in opposite directions as shown schematicallyin FIG. 7B: that is, a stacking order offuel-SOFC-oxygen-SOFC-fuel-SOFC, etc. There are no separate interconnectpieces, and the gas-flow volume per unit generator area is reduced by afactor of ≈2.

[0083]FIGS. 7A and 7B illustrate a straightforward approach in which theindividual planar integrated stack pieces 12 are mounted between endplates 24 and feed tubes 26, requiring seals between the stack piecesand end plates. An alternative approach is the deposit of cells on bothsides of unistructure closed-end flattened tubes 26, using well-knownceramic techniques, eliminating the need for seals, such as those usedin tubular stacks.[S. C. Singhal, Proc. 6th Intl. Symp. Solid Oxide FuelCells, Ed. By S. C. Singhal and M. Dokiya (Electrochemical Society,Pennington, N.J. 1999), p. 39.] On the other hand, the present inventionprovides the high power-to-volume ratios of planar stacks.

[0084] As described above, the SOFC stacking of this invention caninclude patterned layers. As would be understood by those skilled in theart, patterning can be performed during deposition and, as a result, noadditional process steps are required. In particular, masking is readilyincorporated and used with sputter deposition. Colloidal spraydeposition processes are also contemplated in conjunction with contactmasks, especially ultrasonic sprays which produce very small (20-40 μm)droplets.[A. Q. Pham, T. H. Lee, and R. S. Glass, Proc. 6th Intl. Symp.Solid Oxide Fuel Cells, Ed. By S. C. Singhal and M. Dokiya(Electrochemical Society, Pennington, N.J. 1999), p. 172.] The design,assembly and resulting configuration of this invention can provideseveral distinct advantages:

[0085] 1. Improved mechanical strength and toughness. The devices aredeposited onto an electrically insulating ceramic that can be chosen fordesired strength, toughness, thermal conductivity, and thermal expansionmatch to cell components. Partially-stabilized zirconia is a preferredchoice based on its high toughness [T. Yoshida, T. Hoshina, I.Mukaizawa, and S. Sakurada, J. Electrochemical Society, 146, 2604(1989)] and good thermal expansion match. Other materials with lowerelectrical conductivity and higher thermal conductivity may also beadvantageous.

[0086] 2. Simplified gas manifolding and sealing. The stacking geometryshown in FIG. 1 would require seals like other planar stacks. However, aflattened tube design allows simple gas manifolding as in varioustubular stacks of the prior art.[S. C. Singhal, Proc. 6th Intl. Symp.Solid Oxide Fuel Cells, Ed. By S. C. Singhal and M. Dokiya(Electrochemical Society, Pennington, N.J., 1999.] That is, one gas isintroduced through a feed tube into the closed-end flattened tube, andthe other is fed to the outside. No sealing is required, or a singleseal at the tube end can be used.

[0087] 3. Small volume and weight. The design eliminates the separateinterconnect pieces present in most planar stacks, and at the same timereduces the number of gas-flow channels by a factor of two. Thesechanges reduce the volume and weight required to generate a given amountof power.

[0088] 4. Reduced internal electrical losses. The integrated stackfeatures intimate contact of electrodes with interconnects, eliminatingthe resistance loss associated with the pressure contacts used in manyplanar stacks.[B. Krogh, M. Brustad, M. Dahle, J. L. Eilertsen, and R.Edegard, Proceedings of the 5th International Symposium on Solid OxideFuel Cells, Ed. By U. Stimming, S. C. Singhal, H. Tagawa, and W.Lehnert, (Electrochemical Society, Pennington, 1997) p. 1234.] Whilepressure contacts to both ends of the integrated stack elements arerequired for current collection, the subdivision of the cell area into alarge number of small series-connected cells results in relatively lowcurrents and high voltages, such that losses at these contacts areminimal. Compare, for example, a 10 cm×10 cm single cell with the samesize integrated stack element. (See, FIG. 8.) Assuming a single cellproducing 0.5 W/cm² at an operating voltage of 0.7V, the total currentis 70A. On the other hand, the same area integrated stack with 452-mm-wide cells and 0.2-mm-wide interconnects will run at 22.5 V and≈2A. Assuming 2-mm-wide current collector strips at either end of theintegrated stack, the collector current density would be the same as fora conventional cell. Thus, an ≈1 Ω cm² contact resistance would yield≈0.5V drop at the contact: this would roughly halve the power densityfrom the single cell, but would have negligible effect on the integratedstack.

[0089] Another consideration is the lateral resistance loss across theelectrodes. The resistance loss is given by ρL²/2t_(e), where ρ is theelectrode resistivity, L is the length of the current path (i.e. thecell width), and t_(e) is the electrode thickness. This relationshipclearly shows that small cell widths minimize ohmic losses. For example,a LSM cathode (p≈10⁻² Ω cm) with L=2 mm and t_(e)=20 μm yields 0.1Ω-cm². This value is reasonably low even for this very thin electrode,and can be reduced further by reducing the cell width or increasingthickness. (See, FIG. 9.)

[0090] 5. The unit cell size is very small in one direction, ≈2 mm, andis several cm in the other. Scaleup of the stacks would involveprimarily increasing tube length and number of cells, and perhaps aslight widening of the tubes, such that there would be little change inthe unit cell size. This will be very favorable in the scaleup stage ofstack development, compared with conventional planar stacks where theindividual cell area must be increased.

[0091] 6. This invention provides considerable flexibility in stackdesign. For example, stack performance can be optimized by varyingindividual cell widths along the gas flow direction. The variation wouldbe used to compensate for variations in gas composition and celltemperature along the length of the tubes.[See, for example, H. Yakabe,T. Ogiwara, I. Yasuda, and M. Hishinuma, Proc. 6th Intl. Symp. SolidOxide Fuel Cells, Ed. by S. C. Singhal and M. Dokiya (ElectrochemicalSociety, Pennington, N.J. 1999), p. 1087.] For example, since alower-temperature region would likely have lower current densities, thecell widths would be increased slightly to retain the same current percell.

[0092] As mentioned above, the present invention includes fabricationand/or methods for the construction and assembly of the stackinggeometries and configurations described herein. As a point of contrastand comparison, consider techniques of the prior art: a tubular versionof series-interconnected cells, where individual cells and interconnectswere deposited in bands around calcia-stabilized zirconia (CSZ) supporttubes. A major difficulty was obtaining patterned deposits with methodssuch as thick-film slurry coating and electrochemical vapor deposition(EVD). For example, a Ni-YSZ layer was slurry coated, then patterned bychemical etching. Electrolyte and interconnect layers were patterned byfirst covering selected areas with a loosely-bound powder, depositingthe coating, and then mechanically scraping away the powder and theattached thick-film deposit. Disadvantages of these methods include thedifficulty in scaling them into reliable mass production steps, the poorspatial resolution that led to relatively large unit cell sizes (1 cm),and the cycling to high temperature (≈1500° C.) required for eachdeposition step.

[0093] Techniques recently developed for preparing low-to-mediumtemperature SOFCs, which typically involve thin (5-10 μm) electrolytes,are ideally suited for and can be readily applied to making thecomponents, composites and stacks of this invention. Thin electrolytesand electrodes have been prepared using various colloidal deposition andsintering processes,[K. Z. Fung, J. Chen, C. Tanner, and A. V. Virkar,Proc. 4th Intl. Symp. Solid Oxide Fuel Cells, Ed. by M. Dokiya, O.Yamamoto, H. Tagawa, and S. C. Singhal (Electrochemical Society,Pennington, 1995) p. 676. S. deSouza, J. S. Visco and L. C. DeJonghe,Solid State Ionics, 98, 57 (1997)]. Tape calendaring and sintering, andsputter deposition methods are also well-known, reported in thepertinent literature and can be used to prepare such components. FIG.12, for example, shows a cross-sectional SEM image of a typicalsputter-deposited cell of the type available through prior artfabrication and now applicable to this invention, where the YSZelectrolyte and Ni-YSZ electrode were sputter deposited using suchtechiques. It is well known that sputter deposition can producepatterned thin films by depositing through shadow masks. Similarly, someof the colloidal processes such as spray deposition also can be used forpatterned deposition.

[0094] Shadow masking is a simple technique for making patterneddeposits. Mechanisms for accurately aligning shadow masks over planarsubstrates are readily available. As discussed above, it is desirable tohave relatively small width cells. Feature sizes down to 0.1 mm areeasily achieved, such that relatively small (e.g. 1-2 mm wide) cells canbe prepared. Using the patterning techniques employed for banded-tublarSOFCs, it was more difficult to achieve the desired size reduction; 1-cmwide cells were used in those stacks, but this size does not provide thelow ohmic loss observed with the present invention.

[0095] Cell component materials in the integrated stacks, geometries andconfigurations of this invention can be largely the same as those usedin conventional SOFCs: YSZ electrolyte, LSM cathode, and Ni-YSZ anode.The same type of variations used to improve performance, e.g. additionof YSZ to the LSM cathode or ceria to the anode, can be employed, suchvariations including those discussed elsewhere herein. The interconnectmaterial can be doped LaCrO₃ (LSC). As the interconnect conductivityrequired in an inventive stack is much lower than in conventional stacks(the interconnect layer is only several microns thick), alternatematerials with lower conductivity than LSC can be used. Reference ismade to Example 13 and FIG. 11.

[0096] An important aspect of an integrated stack is the supportmaterial. In prior work on the tubular banded cells, the support wascalcia-stabilized zirconia, providing the advantage of excellent thermalexpansion match with the YSZ electrolyte and other cell materials.However, the ionic conductivity was a problem because it tended to shuntthe cells. In response thereto, another feature of this invention,distinct from the banded-tubular prior art, is low operatingtemperature, which minimizes support shunting. Another “problem” withzirconia is the poor thermal conductivity, which limits thermal shockresistance. Accordingly, support components of this invention cancomprise materials overcoming such concerns, including but not limitedto MgO and PSZ, partially-stabilized zironia.

[0097] As mentioned above, the present invention could be used in anyfuel cell application, including the full range of commercialapplications which can be expected of fuel cells. A particularapplication of this stacking approach is for making small fuel cellpower supplies for portable applications, especially those incorporatingthe low temperature cells of the type described elsewhere herein.

EXAMPLES OF THE INVENTION

[0098] The following non-limiting examples, data and referenced graphicsillustrate various aspects and futures relating to the cell apparatus,assemblies, configurations and methods of this invention, including thesurprising and unexpected results showing utility of hydrocarbon fuelsat low temperature without carbon deposition. Comparable utilities andadvantages can be realized using other embodiments, consistent with thisinvention. For instance, while numerous examples illustrate thisinvention through use of methane and ethane, other hydrocarbonsincluding the corresponding alcohols can be used with equal effectthrough straight-forward modifications of this invention and relatedfuel cell components, such modifications as would be well known to thoseskilled in the art and made aware of this invention.

[0099] The SOFCs used in several examples were fabricated usingtechniques well-known in the art. In particular, several embodimentswere utilized, but comparable results are available using other suchelectrodes and electrolytes, including those described herein. In somepreferred embodiments for use with several examples, SOFCs werefabricated on porous La_(0.8)Sr_(0.2)MnO₃ (LSM) cathodes. The LSMpellets were ≈2 cm in diameter and 1 mm thick, and were produced usingstandard ceramic processing techniques. All SOFC layers, starting with a0.5 μm-thick (Y₂O₃)_(0.15)(CeO₂)_(0.85) (YDC) porous film, weredeposited on the LSM pellet using dc reactive magnetron sputtering. Theelectrolyte, 8 mol % Y₂O₃-stabilized ZrO₂ (YSZ), was subsequentlydeposited under conditions yielding a dense 8 μm thick film. To completethe cell, another 0.5 μm-thick YDC film was deposited, followed by aporous, 2 μm-thick Ni-YSZ anode.

[0100] Anode reactions were studied using impedance spectroscopy withanodes that were sputter deposited onto both sides of bulk YSZsingle-crystal electrolytes. Data was collected in various fuelenvironments at the indicated temperatures.

Example 1

[0101] Single-cell current-voltage measurements were carried out in airand methane. Identical results were obtained for pure and humidified(containing 3% H₂O) methane in the early stages of cell tests; whilecell performance was stable with dry methane, after ≈2 hrs of testing inhumidified methane, cell performance degraded because of oxidation ofthe anode Ni. FIG. 1 shows the measured current density and powerdensity vs. voltage. The open-circuit voltage (OCV) was 1.06V. Thecurrent density vs. voltage curves were non-ohmic indicating asubstantial electrode overpotential. Based on prior studies of thesecells operated on hydrogen fuel, the current densities were limitedprimarily by cathode, overpotential. Current densities increased withincreasing temperature, such that maximum power density increased from250 mW/cm² at 600° C. to 370 mW/cm² at 650° C.

Example 2

[0102] By way of comparison, the results of Example 1 were similar tothose obtained for cells operated with humidified hydrogen fuel, exceptthat the power densities were ≈20% greater. Visual observation, energydispersive x-ray (EDX), and scanning electron microscopy (SEM)observations of the anodes, carried out after the call tests, showed noevidence of carbon deposition after ≈100 hrs of operation.

Example 3

[0103] Successful cell operation on dry methane (Example 1) indicatedthat direct electrochemical oxidation, CH₄+2O₂=2H₂O+CO₂ ΔG°(600°C.)=−800 kJ/mol (3) was the primary anode reaction mechanism. The OCVvalues measured in Example 1, ≈1.06V, were typically very close to thevalues measured for these cells operated with 97% H₂+3% H₂O fuel. Thisis reasonable given that the ΔG°(600° C.) values are similar for eqn. 3and hydrogen oxidation. The exact Nernst potential for eqn. 3 cannot becalculated since the, H₂O and CO₂ partial pressures are not known, butthe measured OCV_(s) suggest reasonable values, <0.18 atm for H₂O and<0.09 atm. CO₂.

[0104] While eqn. 3 shows electrochemical oxidation of methane, itshould be noted that the present invention contemplates the possibilityof other reaction mechanisms for the desired oxidation products. Forinstance, various intermediate reactions and/or species can existenroute to complete oxidation under cell operating conditions. Whileeqn. 3 illustrates methane oxidation, equations can be provided toportray oxidation of other hydrocarbons discussed herein, likewiseaccounting for and including the presence of various other intermediatereaction mechanisms and/or species.

Example 4

[0105] It is unlikely that hydrocarbon reforming played a role in thecell operation of Example 1, and if so, only after H₂O and CO₂ wereproduced by reaction (3). Discounting such an occurrence, reformingrates were probably too low to contribute significantly to the anodereaction, because of the small anode area (≈1 cm²) and low temperature.Furthermore, the relatively high fuel flow rates used invariably flushany reformation reactants and products from the anode compartment.

Example 5

[0106] Direct hydrocarbon oxidation is further evidenced by impedancespectra (FIG. 2) obtained from the above-described Ni-YSZ/YDC anodes inhumidified methane (a) and humidified dilute H₂ (b). An 3% H₂+3% H₂O+94%Ar mixture, which was used to simulate a slightly reformed methane fuel(b), yielded electrode arcs with a much different shape than those formethane, indicating that the primary anode reaction with methane was notoxidation of hydrogen produced by reforming. Also shown in FIG. 2 is theimpedance result for the anode operated in 97% H₂+3% H₂O (c). Thiselectrode arc was much smaller than that for methane, indicating a loweranode overpotential and explaining why the SOFC current densities werehigher for hydrogen than methane.

Example 6

[0107] It is thought that one factor contributing to the rapid directelectrochemical oxidation of methane at these temperatures is the anodesemployed in the SOFCs and, in particular a combination of Ni-YSZ and YDClayers. This is illustrated in FIG. 3, which compares the impedancespectra taken in humidified methane for Ni-YSZ/YDC and Ni-YSZ anodes.The YDC layer causes a factor of ≈6 decrease in the electroderesistance. This observation is consistent with prior studies indicatingthat ceria promotes hydrocarbon oxidation. Without restriction to anyone theory or mode of operation, ceria is believed to be beneficial forseveral reasons. First, it becomes a mixed conductor in a reducing fuelenvironment, a condition which should expand the reaction zone beyondthree-phase boundaries. Second, the ionic conductivity of ceria ishigher than that of YSZ, which improves the transport of oxygen ionsfrom the electrolyte to the anodes. Third, ceria is known to readilystore and transfer oxygen. The present invention also indicates that theoxygen storage capability of ceria can be enhanced by the addition ofzirconia. Preferred embodiments include anodes with two ceria/zirconiainterfaces where enhanced oxygen storage can, in this manner, increasemethane/hydrocarbon oxidation rates.

Example 7

[0108] Another result observed from the cell tests of this invention wasthe absence of carbon deposition. In general, carbon deposition canoccur by methane pyrolysis,

CH₄=C+2H₂,  (4)

[0109] or disproportionation,

2CO=C+CO₂,  (5)

[0110] On the other hand, the oxygen flux arriving at the anode duringSOFC operation tends to react with any carbon, suppressing carbondeposition. The role of methane pyrolysis was tested by flowing puremethane over SOFC anodes, without SOFC operation, such that no reactionproducts were present. No carbon deposition was observed at <700° C.,and the amount of carbon deposited increased only with increasingtemperature above 700° C., showing carbon deposition via methanepyrolysis does not occur readily at low temperatures.

Example 8

[0111] During cell operation, product gases are present in the anodecompartment, raising, at least, the possibility of carbon deposition viareaction (5). However, as noted above, the present cell tests providedata for nearly pure hydrocarbon in the anode compartment, perhaps dueto the small-area cells and relatively high fuel flow rates resulting insmall concentrations of reaction products. Even so, consideration wasgiven to the situation encountered in a SOFC stack, where the productsof reactions (1)-(3) would be present at substantial partial pressures.Thus, a simple equilibrium calculation was done to determine theconditions where one might expect carbon deposition free stack operationin CO-CO₂ mixtures. FIG. 4 shows the CO/CO₂ partial pressure ratio atequilibrium with graphitic carbon (eqn. 5) vs. temperature, for twoCO+CO₂ total pressures. For sufficiently low CO/CO₂ ratios, carbondeposition will not occur. The optimal temperature range for SOFC stackoperation on dry methane is ≈500-700° C. If the temperature is <500° C.,carbon deposition proceeds by reaction (5) unless CO/CO₂ ratios are verylow. While temperatures ≧700° C. would tend to suppress carbondeposition by reaction (5), they would allow carbon deposition by directpyrolysis of methane (eqn. 4).

Example 9

[0112] Some internal reforming would be necessary in a SOFC stack toproduce a small amount of CO and H₂. However, carbon monoxide andhydrogen gas would balance the CO₂ and H₂O produced by direct oxidation,preventing exceedingly low CO/CO₂ and H₂/H₂O ratios where the anode Nimay oxidize.

Example 10

[0113] Ethane fuel reactions were studied in SOFC's with anodes of lowNi content. Various cells were made using Ni-YDC anodes containing 10,20 and 40 weight percent Ni. The porous Ni-YDC anodes were ≈2 μm-thick.The cell structure and processing was similar to that described above,except that the cathode was an LSM-YSZ mixture. In order to identifycarbon-free operating conditions, the Ni-YDC anodes were tested inethane fuel environments at temperatures ranging from 500-700° C.

[0114] Ethane oxidation was studied at Ni-YDC anodes using low Niconcentrations intended to limit carbon deposition. FIG. 5 summarizescarbon deposition results at the various Ni-YDC anodes exposed to dryand wet (3% H₂O) ethane from 500-600° C. These reaction studies weremade without cell operation and indicate the onset of carbon depositionfor the given conditions. In wet ethane, anodes with lower Ni content(10-20 wt %) were more resistant to carbon deposition at highertemperatures. For dry ethane fuel, carbon deposition occurred even forlow temperatures and anodes with low Ni content. These observations showthat carbon-deposition-free cell operation can be conducted using wetethane with any of the above anode compositions at 500° C., and up to550° C. for anodes with 10-20wt % Ni.

Example 11

[0115] Current voltage measurements of cells were taken under operatingconditions that avoided carbon deposition. The performance of a celloperating with wet (3% H₂O) ethane fuel at 500° C. (▴) is illustrated inFIG. 6 and compared to wet hydrogen 92% hydrogen () and humidifieddilute (94% Ar) hydrogen (▪). In this case the Ni-YDC anode contained40wt % Ni. The cell current densities were relatively low because of thelow temperature. Current densities for ethane fuel were typically about35% less than for hydrogen fuel. A maximum of ≈35 mW/cm² was obtainedwith ethane. No carbon deposition was detected. Carbon-deposition-freecell operation with anodes of lower Ni content (10-20 wt %) was achievedup to 600° C., which is beyond the temperature range indicated by thecarbon reaction studies given in FIG. 5. This suggests that the oxygenflux arriving at the anode during cell operation reacted with anycarbon, thereby suppressing carbon deposition. However, cell powerdensities were somewhat low in these cells, either because of low anodeelectrical conductivity or low Ni catalyst content.

Example 12

[0116] With reference to FIGS. 7A and 7B, the solid oxide fuel cellcomponents of this invention can be stacked as shown in FIG. 10.Flattened support tubes allow high volume densities and minimize sealingproblems. The integrated configurations eliminate interconnects andreduce the number of flow fields by two, relative to planar fuel cellsof the prior art. Because the integrated arrays are not in contact, heatconductors can be inserted therebetween; this can help eliminate thermalgradients and improve efficiency through reduction of coolingrequirements. Current flows along the surfaces of the fuel cell ratherthan between separate components thereof.

Example 13

[0117] With reference to FIGS. 7A, 7B and 10, the fuel cell componentsand material composition thereof can be selected according toperformance and function parameters desired. FIG. 11 providesconstruction profiles of two such fuel cells of this invention, asdesigned according to desired/relative operating temperatures. Suchcells, in accordance with the broader aspects of this invention, can beintegrated as described herein and incorporated into an assembly of suchcells. Various other material choices and component constructions wouldbe well-known to those skilled in the art and made aware of thisinvention, depending upon a particular performance property or parameterdesired. Such designs and constructions are limited only by theavailability of a desired component material to be used in conjunctionwith thin layer fabrication techniques.

Example 14

[0118] Current procedures can involve first depositing anode material,followed by electrolyte. After high temperature sintering, interconnectand cathode layers are applied and sintered at a lower temperature. ThinNi-YSZ anodes and YSZ electrolytes are deposited using a colloidalprocessing technique called centrifugal casting. In this method, apartially-sintered Ca-stabilized Zirconia (CSZ) ceramic pellet (thesupport for the ISOFC) is immersed in a slurry, based on water oracetone, that contains the appropriate powder. Patterning is achievedsimply by placing an adhesive polymer mask on the pellet. The pellet andslurry are then placed in a centrifuge where centrifugal forces are usedto rapidly deposit the powder onto the pellet surface, yielding highquality, high packing density Ni-YSZ or YSZ coatings. The layers andsupport are co-sintered in air at 1400-1500° C. FIG. 13 shows an opticalmicroscope image of a patterned Ni-YSZ strip after sintering. The layeris uniform with a low-defect-density and well-defined patterned edges.

Example 15

[0119] A key issue in the processing of example 14 is the drying stresscaused by the liquid evaporating from the thin Ni-YSZ and YSZ layers.The film shrinkage during drying can cause cracking, and is generallyworse for thicker films and smaller particle sizes. The processdescribed herein uses nano-scale YSZ for the electrolyte to reducesintering temperatures. It produced several micron thick YSZ films insingle step. Multiple deposition steps were used to make thicker layers.Another aspect of this process is co-sintering. Fuel cells require athin dense YSZ layer with porous cathodes, anodes, and support.Difficulties can include matching shrinkages (to avoid cracking of thelayers and/or sample curvature) and maintaining electrode porosity atthe high temperatures required for YSZ sintering.

[0120] High temperature sintering at 1500° C. resulted in high qualitydense YSZ electrolytes. An SEM image of the top surface of atypical YSZlayer is shown in FIG. 14. The YSZ layer is clearly dense and defectfree, with a large grain size. The shrinkages were matched well: nocracking or curvature of the samples was observed. FIG. 15 shows a SEMimage of a portion of a patterned YSZ film. The YSZ film is seen in oneregion while the other region while the uncovered Ni-YSZ anode can beseen in another region. This result, along with that shown in FIG. 13,demonstrates successful fabrication of patterned anode and electrolytelayers.

Example 16

[0121] The successful fabrication of a patterned anode and electrolyteon a support can be challenging aspects of this type of ISOFCprocessing. Additional steps involve simple slurry painting andrelatively low temperature sintering, using techniques well-known tothose in the art.

[0122] An ISOFC of this invention can, preferably, have a ceramicinterconnect, such as LaCrO₃. However, an Ag paste can also be used asthe interconnect for purposes of illustration. Above ≈600° C., thismaterial sinters to a high density, providing a good gas seal. It iseasily painted onto an ISOFC. A cathode typically comprises standardSOFC materials: (La,Sr)MnO₃ (LSM) or (La,Sr)(Co,Fe)O₃ (LSCF), as can beapplied by painting followed by relatively low temperature sintering.

Example 17

[0123] The centrifugal casting techniques demonstrated in examples 14-15are further illustrated. For use as a support, Ni-YSZ anode substrateswere made according to prior art procedures, mixing YSZ powder (Tosoh)and NiO powder (Baker) in a weight ratio of 1:1. The powder was mixedwith acetone and ball milled for about 30 hrs. 6 wt % starch was thenadded to the mixed powders, which were ball milled for another 2 hrsbefore being dried and pressed into pellets. The pellet diameter was 20mm and thickness was 0.5 mm. An initial calcining was done at 1000° C.for 6 hours.

[0124] Likewise, as described above, a ceramic electrolyte is depositedon a support. A stable YSZ suspension or slurry can be, for instance,prepared by mixing 1 gram of YSZ powder (Tosoh) with 200 ml ethanol andsonicating for 1 hour. In some cases, water can be used. The Ni-YSZsubstrates supports were placed in vessels with flat bottoms and 10 mLof the slurry added along with 10 mL of ethanol (or water). The vesselswere then placed in a centrifuge with a radius of 25 cm, and thecentrifuge was operated for a time and at a rate sufficient toadequately deposit the electrolyte. In this example, the centrifuge wasrun for 30 min at 1500 rpm. In the centrifugal field, the ceramicparticles in the suspension are deposited or forced down to the surfaceof the substrate to form a coating—the supernatant was clear after thecentrifuge process. After decanting the supernatant, the wet green bodywas allowed to dry. The coated supports were dried in open air for about4 hours (or more than 8 hours for water based slurry), then sintered inair at 1000° C. for 4 hours and then at 1400° C. for 4 hours. Similarquality coatings were obtained with water and ethanol.

[0125] Cells can be constructed, as described herein or would otherwisebe known in the art to further characterize and test the electrolytedepositions of examples 14-15 and 17.

Example 18

[0126] To that effect, La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃(LSCF)-Ce_(0.9)Gd_(0.1)O_(1.95) (GDC) cathodes were prepared by mixingLSCF powder (Praxair) with GDC powder (NexTech) in the weight ratio of1:1. For purposes of comparison, La_(0.8)Sr_(0.2)MnO₃ (LSM)-YSZ cathodeswere prepared by mixing LSM powder (Praxair) with YSZ powder (TOSOH),also in the weight ratio of 1:1. For both cathodes, a screen printingvehicle (Heraeus V737) was added to the mixed powder to make a slurry.The slurry was applied onto the YSZ electrolyte coating side of theaforementioned support then fired at 400° C. for 2 h to burn out thebinder. Then, a second layer of the cathode slurry was applied, followedagain by burnout at 400° C. for 2 h. In order to obtain good currentcollection, two more layers of pure LSCF (or LSM) were applied on top ofa Pt mesh, that became embedded in the cathode. Finally, the respectivecomposites were sintered: at 900° C. for 3 hrs, for LSCF-GDC, and 1100°C. for 3 hrs for LSM-YSZ. The cathode area for each was 0.5 cm².

[0127] The anode sides of the cells were attached and sealed to one endof alumina tubes by applying silver paste (DAD-87, Shanghai ResearchInstitute of Synthetic Resin). Cell tests were carried out by flowingH₂+3% H₂O at 30 ml/min to the anode side while the cathode was exposedto ambient air. Impedance measurements were carried out during celltests using a Solartron-1260 Impedance Analyzer. The frequency range was1-10⁶ Hz. SEM images were obtained in a Hitachi 3500 microscope.

Example 19

[0128]FIG. 16 shows a fracture cross-sectional SEM image of a SOFC witha LSM-YSZ cathode after cell testing. It can be clearly seen that thecentrifugal cast YSZ coating of this invention is dense, with athickness of 25 μm. The anode support shows a good uniform porosity. Atthe interface of the electrolyte and the anode, the contact is verygood. The cathode is also porous but shows some large pores along with afiner-scale porosity. The cathode thickness was somewhat non-uniform.

Example 20

[0129]FIG. 17 shows the performance (hydrogen fuel) of an anodesupported SOFC with a LSCF-GDC cathode. The measured open circuitvoltage (OCV) was within 30 mV of the value predicted using the Nernstequation, indicating that the deposited YSZ electrolyte was fully denseand free of pinholes. The maximum power density of this SOFC was0.93W/cm² at 800° C. The I-V curves showed positive curvature,particularly at lower temperatures. This is similar to prior artliterature reports on anode supported cells operated on hydrogen. Adetailed study of anode supported cells has shown that cell powerdensities are limited by electrode activation and/or concentrationpolarization. The fact that negative curvature is not observed in FIG. 1is partly due to the fact that the current was not measured beyond 2A/cm², but also indicates that the Ni-YSZ anode supports had reasonablyhigh porosity. The cell performed stably for 90 hrs at 700° C. at 0.6Vand 0.35 W/cm², and was still working well when the test was stopped.

Example 21

[0130]FIG. 18 shows the performance (hydrogen fuel) of a SOFC usingLSM-YSZ as the cathode. The maximum power densities were slightly higherfor LSM-YSZ than for LSCF-GDC, e.g. 0.88 W/cm² at 800° C. This issomewhat surprising given that a number of impedance spectroscopystudies have indicated that LSCF and LSCF-GDC provide substantiallylower low-current polarization resistance than LSM-YSZ. FIG. 19 comparesthe maximum power densities for the present SOFCs with those attainableusing cells (Y) of the prior art. It can be seen that the present cellsprovide comparable or somewhat lower power densities than these priorreports.

Example 22

[0131] Impedance spectra taken during a cell test (hydrogen fuel) atopen circuit are shown in FIG. 20. The semi-circular plots, representingthe combined anode and cathode interfacial resistances, decreasedrapidly in size with increasing temperature, as expected. The leftreal-axis intercepts of the semicircles, representing the ohmic lossesin the cell tests (given in Table 1), also decreased with increasingtemperature. As can be seen from Table 1, the resistance loss of theSOFC mainly came from polarization (>80%), even though the electrolytethickness was relatively large, 25 μm. As discussed previously, thisloss is believed to arise primarily from the cathode. The ohmicresistance measured from the impedance data agrees fairly well with theknown resistivity of YSZ. For example, at 800° C. the impedance datayields 0.11 Ω cm². Given the deposited YSZ coating thickness of 25 μm,this corresponds to a conductivity of 0.23 S/cm. This comparesreasonably well with literature YSZ conductivity values at 800° C. of0.055 and 0.02 Scm⁻¹. Also shown in Table 1 is the low-current cellresistance obtained from the slope of the I-V characteristics in FIG.17. This total cell resistance, which includes both the ohmicresistances and interfacial polarization losses, agrees well with thesum of these resistances measured from the impedance data. TABLE 1 Theresistance distribution of the fuel cell Temperature (° C.) 600 650 700750 800 R_(wc) (Ωcm²)^(a) 2.9 1.8 1.2 0.77 0.63 R_(ohmic) (Ωcm²)^(e)0.38 0.22 0.16 0.13 0.11 R_(interface) (Ωcm²)^(d) 2.52 1.58 1.04 0.640.52

[0132] The results of examples 17-22 show that conventional SOFCs can beprepared using centrifugal casting of YSZ electrolytes, in particular onNi-YSZ anodes/supports. After application of a suitable cathode, theSOFC performance is comparable to that of similar cells prepared usingother methods. Resistance loss analysis based on impedance spectroscopyindicated that the main loss came from polarization. The performance ofSOFCs with LSCF-GDC and LSM-YSZ cathodes was similar.

[0133] While other colloidal processing methods are available, there aresome potential advantages of the present centrifuge technique. Highinitial coating packing density and low defect are realized due to thecentrifugal force, so that high quality coatings are reliably obtainedwithout the need for a clean room environment. Water or alcohols can beused to make the suspensions, and the raw materials are usedefficiently, so the process is environmentally friendly. The techniqueis simple, fast, and economical. The coating thickness can be easily andaccurately controlled over a large dimensional range.

Example 23

[0134] Cells were prepared and tested as would be known in the art.Conventional Ni-YSZ anode substrates can be fabricated, as describedabove, by mixing YSZ powder (Tosoh) and NiO powder (Baker) in a weightratio of 1:1 and pressing the mixed power into pellets with diameter of20 mm and thickness of 0.5 mm. An initial calcining was done at 1000° C.for 6 hours. The YSZ electrolyte layer was deposited by the centrifugalcasting process of this invention, then sintered at 1400° C. for 4hours. The resulting YSZ thickness was ≈25 μm.

[0135] Two kinds of cathodes were, again, used. La_(0.8)Sr_(0.2)MnO₃(LSM)-YSZ cathodes were prepared by mixing LSM powder (Praxair) with YSZpowder (TOSOH) in the weight ratio of 1:1.La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ (LSCF)-Ce_(0.9)Gd_(0.1)O_(1.95) (GDC)cathodes were prepared by mixing LSCF powder (Praxair) with GDC powder(NexTech), also in the weight ratio of 1:1. A slurry was prepared bycombining the mixed powders with a binder (Heraeus V737), and applyingit to the YSZ electrolyte. A Pt mesh current collector was embedded inthe cathode. Finally, the cathodes were sintered at 900° C. for 3 hrsfor LSCF-GDC, and at 1250° C. for 1 hr for LSM-YSZ. The cathode area foreach cell was 0.5 cm².

[0136] The anode sides of the cells were attached to alumina tubes andsealed by applying silver paste (DAD-87, Shanghai Research Institute ofSynthetic Resins). Cell tests were carried out by flowing humidified (3%H₂O) fuel, either pure methane or natural gas, at ≈30 ml/min to theanode side. The cathode was exposed to ambient air.

Example 24

[0137]FIGS. 21 and 22 show the performance of SOFCs with LSM-YSZcathodes in humidified methane and natural gas, respectively, taken inthe first few hours of operation. Open circuit voltages increased from≈1.1 to 1.17V with increasing temperature. Table 2 shows the powerdensities at 0.7V for these cells, and a similar cell with a LSCF-GDCcathode, for hydrogen, methane, and natural gas. (See, also, thecomparative data of example 6, demonstrating use of a conventionalNi-YSZ SOFC in conjunction with a hydrocarbon fuel.) Power densities formethane and natural gas were similar, and ≈10% lower than for hydrogen.The highest power density obtained with methane was 0.96 W/cm² at 800°C., for the cell shown in FIG. 21. The maximum power densities weresimilar for SOFCs with LSM-YSZ and LSCF-GDC cathodes, and dropped withdecreasing temperature. TABLE 2 Power density at 0.7 V for SOFCs withLSM-YSZ and LSCF-GDC cathodes in hydrogen, methane, and natural gasLSM-YSZ LSCF-GDC T(C) H₂ CH₄ Natural Gas H₂ CH₄ 600 0.12 0.10 0.09 0.120.08 650 0.30 0.15 0.15 0.21 0.15 700 0.45 0.29 0.29 0.33 0.28 750 0.620.55 0.52 0.53 0.47 800 0.86 0.89 0.89 0.75 0.73

Example 25

[0138] The I-V curves showed positive curvature, particularly at lowertemperatures, similar to reports of anode-supported cells operated onhydrogen. A prior art study of anode supported cells run with hydrogenshowed that cell power densities can be limited by a number of factors,including concentration polarization. J. W. Kim, A. V. Virkar, K. Z.Fung, K. Mehta, and S. C. Singhal, J. Electrochem. Soc., 146 (1) 69-78(1999). The modest decrease in cell power density for methane relativeto hydrogen may be related to the higher mass of methane molecules,which yields slower gas-phase diffusion and increased concentrationpolarization. Note, however, that each methane molecule reacts with fourtimes as many oxygen ions as each hydrogen molecule: less methanegas-phase diffusion is needed to yield the same cell current. Anotherpossible explanation is a higher polarization resistance associated withslower electrochemical oxidation of methane versus hydrogen.

Example 26

[0139] The cells showed good stability with methane and natural gas. Acell with a LSCF-GDC cathode was operated for >90 hours with methane,before the test was stopped with the cell still running well. FIG. 23shows current density versus time. The cell was kept at 0.6V and 700° C.for most of the test, and the current density was in the range from0.6-0.67 A/cm². The power density increased slightly during the first≈20 hours of the test, and then stabilized for ≈20 hours beforedecreasing slightly over the last 50 hours. There were frequent breaksduring the test when current-voltage characteristics were measured atdifferent temperatures. During the test operation, fuel flow wasaccidentally stopped for ≈10 min. While the cell recovered to nearly itsinitial performance, the onset of the gradual decrease in performancecoincided with this event.

[0140] Another SOFC with a LSM-YSZ cathode was operated in humidifiednatural gas for >40 h at 0.6V. The natural gas performance was similarto that for methane, showing an initial increase and then a gradualdecrease, with a current density of ≈0.5 A/cm².

Example 27

[0141] After these tests, SEM-EDX measurements taken at various pointson the Ni-YSZ anodes showed only very small C peaks. FIG. 24 shows atypical result obtained after the test shown in FIG. 23. A small C peakis present; for comparison, anodes with visible carbon yielded a peakthat was comparable in size to the Zr peak. For natural gas, carbon wasobserved visually on the Pt paint used to attach the current collectorwires to the anode.

[0142] While the cells were quite stable in methane during operation,there were some conditions seemed to induce carbon deposition. Forexample, at 800° C. and open circuit condition, carbon depositionpresumably caused the cell to crack after ≈10 mins. In general, it wasfound that the cells could be kept at open-circuit conditions for >10minutes at 700° C., and longer times at lower temperatures, withoutdeleterious effect. This is in reasonable agreement with measurementsshowing that the carbon deposition rate on Ni-YSZ becomes quite lowbelow ≈650° C., and confirms coking as a potential problem fordirect-methane operation of anode-supported SOFC stacks, especially ifthe operating temperature is above ≈700° C. C. M. Finnerty, N. J. Coe,R. H. Cunningham, and R. M. Ormerod, Catalysis Today 46 (1998) 137.

Example 28

[0143] Open circuit voltages (OCV) versus temperature, measured from thecell test results with hydrogen and methane, are plotted in FIG. 25. TheOCV values for methane increased linearly with increasing temperature.For comparison, the same cell operated with humidified hydrogen showedthe expected OCV decrease with increasing temperature. The experimentalresult for hydrogen is ≈25 mV lower than the theoretical predictionshown, presumably due to experimental effects such as leakage at theseals. The theoretical OCV calculated from the Nernst equation, assuminga number of different possible methane electrochemical reactions, isalso shown for comparison in FIG. 25. Possible reactions consideredinclude direct electrochemical oxidation of methane, or the oxidation ofH₂, CO, or solid C produced by initial reactions such as reforming ormethane cracking. In the calculation, the partial pressures of CH₄ andH₂ were taken to be 1 atm, while the partial pressures of CO, CO₂, andH₂O were taken to be 0.03 atm. (This assumption most likelyoverestimates the partial pressures of CO and CO₂, since they were notintentionally added to the fuel. Note, for example, that a factor-of-10lower CO partial pressure would lead to a predicted OCV ≈40 mV higherthan shown in FIG. 24 for C+1/2O₂=CO.) Without limitation to any onetheory or mode of operation, the OCV versus temperature data agrees wellwith the prediction for partial oxidation of C. While there isuncertainty in the predicted absolute OCV values as noted above, it isclear that only C partial oxidation yields a slope near that observedexperimentally. While the results in FIG. 25 are interesting, moreevidence is needed before making a definitive conclusion about anyparticular reaction mechanism.

Example 29

[0144] The data and results of the preceding examples are reasonablyconsistent with prior work on direct methane SOFCs. The OCV of 1.1 to1.17 V measured at 600-800° C. is between the values measured at 1000°C., 1.2V, and at 550-650° C., 1.05V. See, respectively, K. Ukai, Y.Mizutani, Y. Kume, and O. Yamamoto, in: H. Yokokawa, S. C. Singhal(Eds.), Solid Oxide Fuel Cells VII, Electrochemical Society, Pennington,N.J., 2001, p. 375 and E. P. Murray, T. Tsai, and S. A. Barnett, Nature400 (1999) 649. The present power densities are the highest reported fora YSZ thin electrolyte SOFC operated with methane at ≦800° C., but arelower than the value reported at 1000° C. for a thick SOFC electrolyteSOFC, 2.0W/cm². Prior work on thin electrolyte SOFCs operated withmethane was done at lower temperatures, so it is not surprising that thepresent power densities are higher. The present invention demonstratesthat Ni-based anodes can be used substantially without deleteriouscoking at higher temperature, preferably with a substantial currentflowing through the cells.

[0145] While the principles of this invention have been described inconnection with specific embodiments, it should be understood clearlythat these descriptions, along with the chosen figures, charts,graphics, and data therein, are made only by way of example and are notintended to limit the scope of this invention, in any manner. Forexample, the inventive anodes and related cellular components have beenshown as utilized with various hydrocarbons; however, as would bewell-known to those skilled in the art and made aware of this invention,the methods described herein can also be utilized with various otherhigher molecular weight hydrocarbon stocks or reaction systems.Likewise, while certain ceria and/or electrolyte materials have beendescribed herein, others can be used alone or in combination and with orwithout various dopants to achieve the same or similar effect. Whilevarious parameters, such as temperature and concentrations, have beendescribed in conjunction with the construction, fabrication and/oroperation of various fuel cells and their components, the sameparameters can be varied in order to achieve oxidation rates and/orpower densities comparable to those described herein. Other advantagesand features of this invention will become apparent on the followingclaims, with the scope thereof determined by the reasonable equivalents,as would be understood by those skilled in the art.

1. A method of using a hydrocarbon fuel to operate a solid oxide fuelcell, said method comprising: providing a solid oxide fuel cell havingan anode comprising a catalytic metal; and introducing a hydrocarbonfuel to said anode, said fuel absent at least one of sufficient carbondioxide and water sufficient to convert said hydrocarbon fuel tohydrogen, said method substantially absent a hydrocarbon reformingstage.
 2. The method of claim 1 wherein said hydrocarbon fuel isselected from the group consisting of about C₁, to about C₁₀hydrocarbons and combinations thereof.
 3. The method of claim 2 whereinsaid fuel comprises methane.
 4. The method of claim 2 wherein said fuelis a natural gas comprising a combination of hydrocarbons.
 5. The methodof claim 1 wherein said metal comprises nickel.
 6. The method of claim 5wherein said anode is a composite of nickel and a doped zirconia.
 7. Themethod of claim 6 wherein said anode is a composite of nickel andyttria-stabilized zirconia.
 8. The method of claim 7 wherein said cellfurther includes a yttria-stabilized zirconia electrolyte on said anode.9. The method of claim 8 wherein said solid oxide fuel cell is operatedat temperatures less than about 800° C.
 10. The method of claim 1wherein said anode and said cell are arranged in a stack of said cells.11. A method of using a hydrocarbon fuel to increase the open circuitvoltage of a solid oxide fuel cell, said method comprising: providing asolid oxide fuel cell having an anode comprising a metal catalytic forhydrocarbon cracking; introducing a hydrocarbon fuel to said anode tooperate said cell; and increasing the operating temperature of said cellabove about 600° C.
 12. The method of claim 11 wherein said hydrocarbonfuel is selected from the group consisting of about C₁ to about C₁₀hydrocarbons and combinations thereof.
 13. The method of claim 12wherein said fuel comprises methane.
 14. The method of claim 12 whereinsaid fuel is a natural gas comprising a combination of hydrocarbons. 15.The method of claim 11 wherein said anode is a composite of nickel and adoped zirconia.
 16. The method of claim 15 wherein said anode is acomposite of nickel and yttria-stabilized zirconia.
 17. A method ofpreparing a solid oxide fuel cell component, said method comprising:preparing a slurry of a component material; providing a supportingsubstrate, said slurry and said supporting substrate in a centrifugeapparatus; and applying centrifugal force to said slurry and saidsupporting substrate, said force sufficient to deposit said componentmaterial on said supporting substrate.
 18. The method of claim 17wherein said substrate is a fuel cell support.
 19. The method of claim17 wherein said substrate is masked to provide a deposition pattern. 20.The method of claim 18 wherein said component material is at least oneof an anode material and an electrolyte material.
 21. The method ofclaim 20 wherein said anode material comprises a composite of nickel andyttria-stabilized zirconia, and said electrolyte material comprisesyttria-stabilized zirconia.
 22. The method of claim 17 wherein saiddeposition comprises multiple applications of centrifugal force.
 23. Themethod of claim 17 further including sintering said deposited componentmaterial.
 24. The method of claim 17 wherein said substrate is an anodeand said slurry comprises an electrolyte material.
 25. A method of usingcentrifugal force to prepare a solid oxide fuel cell component, saidmethod comprising: preparing a slurry of a ceramic electrolyte material;providing an anode substrate, said slurry and said anode substrate in acentrifuge apparatus; and applying centrifugal force to said slurry andsaid anode substrate, said force sufficient to deposit said electrolytematerial on said anode substrate.
 26. The method of claim 25 whereinsaid slurry comprises a liquid selected from the group consisting of analcohol, water, acetone and combinations thereof.
 27. The method ofclaim 25 wherein said anode is a nickel composite.
 28. The method ofclaim 25 wherein said ceramic electrolyte is a yttria-stabilizedzirconia.
 29. The method of claim 28 wherein said anode is a compositeof nickel and yttria-stabilized zirconia.
 30. The method of claim 25wherein said deposited electrolyte is sintered.